1. Field of the Invention
The present invention relates to a method for forming an insulator film on a semiconductor, where a combination of the semiconductor and the insulator film is used in a FET (Field Effect Transistor) or a polycrystal silicon thin film transistor which has a MOS (Metal Oxide Semiconductor) structure. The present invention also relates to a semiconductor device fabricated using the method, and a production apparatus.
2. Description of the Related Art
FETs are widely used for LSIs. To improve the performance of LSIs, there is a demand for a satisfactory thin insulator film which can be formed at a low temperature, and with satisfactory semiconductor-insulator film interfacial quality.
Conventionally, single-crystal silicon is generally thermally oxidized at a temperature of 700xc2x0 C. to 1000xc2x0 C. In thermal oxidation, an oxidation reaction develops from a surface of a semiconductor (a surface of a semiconductor layer) and progresses inward. Therefore, an interface is produced between the semiconductor layer (semiconductor) and an oxide silicon film (gate insulator film) provided by the thermal oxidation of the semiconductor layer surface, i.e., the interface is provided inside the original semiconductor layer. Therefore, the interface is not substantially affected by a condition of the original surface, so that a very satisfactory interface can be advantageously obtained. However, the high temperature process is likely to warp a silicon wafer. Low temperatures suppress warp, but cause an oxidation rate to be rapidly reduced. Thus, a low temperature process is not practical.
An insulator film may also be produced by plasma CVD (Chemical Vapor Deposition), but it is difficult to obtain satisfactory interfacial quality. In this case, the most critical problem is that ion damage due to plasma is inevitable.
On the other hand, the recent development of large-size, high-definition, and high-performance liquid crystal display apparatuses require higher and higher-density TFTs (Thin Film Transistors). There is an increasing demand for TFTs of a polysilicon (poly-Si) film in place of conventional amorphous silicon film TFTs. A gate insulator film, which is crucially important for TFT""s performance and reliability, is provided by plasma CVD. However, when plasma CVD is employed to form a gate insulator film, damage due to plasma is inevitable. In this case, particularly, a threshold voltage of the resultant transistor cannot be controlled with high precision, and reliability of the transistor may be lowered.
As often the case in poly-Si TFTs, a SiO2 film may be formed by plasma CVD using TEOS (Tetra Ethyl Ortho Silicate) and O2 gases. Such a SiO2 film contains carbon atoms which are originally contained in a gas material. Even if the film is formed at 350xc2x0 C. or more, it is difficult to reduce the carbon concentration to 1.1xc3x971020 atoms/cm3 or less. In particular, when the film-forming temperature is as low as about 200xc2x0 C., the carbon concentration in the film is increased by an order of magnitude up to 1.1xc3x971021 atoms/cm3. Therefore, it is difficult to reduce film-forming temperature.
In the case of plasma CVD using SiH4 and N2O-based gases, an interface nitrogen concentration is as great as one atom % or more, so that an interface fixed charge density cannot be 5xc3x971011 cmxe2x88x922 or less. A functional gate insulator film cannot be obtained.
For the purpose of reducing ion damage due to plasma CVD so as to obtain a high-quality insulator film, oxidation methods, such as for example ECR plasma CVD and oxygen plasma, have been developed. However, since plasma is generated in the vicinity of a surface of a semiconductor, it is difficult to fully avoid ion damage.
Cleaning apparatuses using a light source, such as for example a low-pressure mercury lamp and an excimer lamp, have already been brought into mass production.
A method in which light is used to oxidize silicon at a low temperature of 250xc2x0 C. has been studied. In this method, however, a film formation rate is as slow as 0.3 nm/min. At present, it is practically difficult to form an entire gate insulator film (J. Zhang et al., A. P. L., 71(20), 1997, P2964).
Japanese Laid-Open Publication No. 4-326731 discloses an oxidation method which is carried out in an ozone-containing atmosphere. As described below, however, in this method, ozone is produced using light, and the ozone is decomposed using light to produce oxygen atom radicals, i.e., the method comprises two reaction steps. Therefore, the method is poorly efficient, resulting in a low oxidation rate.
As described above, in the case of deposition (plasma CVD, etc.), a thick insulator film can be quickly formed on a semiconductor, but a surface of the original semiconductor remains as an interface between the semiconductor and the insulator film (gate insulator film), and ion damage is inevitable. Therefore, since interface trap density is increased, it is not possible to obtain satisfactory device characteristics.
When an insulator film is formed on a semiconductor using an oxidation method (e.g., an oxygen plasma oxidation method), an oxidation reaction develops from a surface of a semiconductor to the inside, and an interface between a semiconductor layer (semiconductor) and the insulator film is formed inside the original semiconductor layer. Therefore, the interface is not substantially affected by a condition of the original surface, so that a very satisfactory interface can be advantageously obtained. However, the high temperature process is likely to warp a silicon wafer. Low temperatures suppress warp, but cause an oxidation rate to be rapidly reduced. Thus, a low temperature process cannot produce an insulator film at a practical rate.
According to an aspect of the present invention, a method for forming an insulator film at a semiconductor temperature of 600xc2x0 C. or less comprises the steps of forming a first insulator film by oxidizing a surface of a semiconductor in an atmosphere containing oxygen atom radicals, and forming a second insulator film on the first insulator film by deposition without exposing the first insulator film to outside air.
In one embodiment of this invention, the first insulator film forming step comprises generating the oxygen atom radicals by irradiating an atmosphere containing an oxygen gas with light having a wavelength of 175 nm or less.
In one embodiment of this invention, the first insulator film forming step comprises generating the oxygen atom radicals by irradiating an atmosphere containing an oxygen gas with light having a wavelength of 172 nm, the light emitted from a xenon excimer lamp.
In one embodiment of this invention, the first insulator film forming step comprises generating the oxygen atom radicals by irradiating an atmosphere containing an oxygen gas having a partial pressure of 0.05 torr to 50 torr with light having a wavelength of 172 nm, the light emitted from a xenon excimer lamp.
In one embodiment of this invention, the method further comprises, prior to the first insulator film forming step, the step of cleaning the surface of the semiconductor by irradiating the surface of the semiconductor with light having a wavelength of 175 nm or less in an atmosphere having substantially no oxygen.
In one embodiment of this invention, the first insulator film forming step comprises generating the oxygen atom radicals by plasma CVD, wherein there is a predetermined distance or more between a plasma generating site and the surface of the semiconductor.
In one embodiment of this invention, the first insulator film forming step comprises forming the first insulator film where a temperature of the semiconductor is in the range from 100xc2x0 C. to 500xc2x0 C.
In one embodiment of this invention, the first insulator film forming step comprises forming the first insulator film where the formed first insulator film has a thickness in the range from 0.5 nm to 20 nm.
In one embodiment of this invention, the first insulator film forming step comprises mixing the atmosphere with a hydrogen or fluorine gas.
In one embodiment of this invention, the method further comprises performing thermal annealing at the temperature of the semiconductor or less of the first and second insulator film forming steps.
In one embodiment of this invention, the method further comprises subjecting the first insulator film to hydrogen plasma treatment at the temperature of the semiconductor or less of the first and second insulator film forming steps.
In one embodiment of this invention, the second insulator film forming step comprises depositing the second insulator film by plasma CVD.
In one embodiment of this invention, the second insulator film depositing step comprises performing the deposition where a temperature of the semiconductor is in the range from 100xc2x0 C. to 400xc2x0 C.
In one embodiment of this invention, the second insulator film depositing step comprises performing the deposition of the second insulator film using at least silane-based and nitrogen monoxide gases.
In one embodiment of this invention, the second insulator film forming step comprises depositing the second insulator film by photo CVD.
In one embodiment of this invention, the semiconductor is single-crystal silicon.
In one embodiment of this invention, the semiconductor is polycrystal silicon.
In one embodiment of this invention, the semiconductor is a silicon thin film of polycrystal silicon provided on a substrate of at least glass, metal foil, or resin.
According to another aspect of the present invention, a semiconductor device comprises an insulator film formed by the above-described method.
In one embodiment of this invention, the semiconductor device is a silicon thin film transistor.
According to another aspect of the present invention, a semiconductor device comprises a semiconductor comprising a silicon thin film provided on a substrate of glass, metal foil, or plastic, and an oxide film provided on a surface of the semiconductor. An interface between the silicon thin film and the oxide film has a fixed charge density of 1xc3x971011 cmxe2x88x922 or less, an interface trap density of 1xc3x971011 cmxe2x88x922 eVxe2x88x921 or less, and a nitrogen concentration of 1 atom % or less, and the silicon thin film has a carbon concentration of 1xc3x971020 atoms/cm3 or less.
In one embodiment of this invention, the semiconductor device is a silicon thin film transistor.
According to another aspect of the present invention, an apparatus for forming an insulator film on a semiconductor comprises a first reaction chamber for forming a first insulator film by oxidizing a surface of a semiconductor in an atmosphere containing oxygen atom radicals, and a second reaction chamber for forming a second insulator film on the first insulator film by deposition.
In one embodiment of this invention, in the first reaction chamber, the surface of the semiconductor is oxidized with oxygen atom radicals generated by irradiating an atmosphere containing an oxygen gas with light having a wavelength of 175 nm or less.
In one embodiment of this invention, in the first reaction chamber, the surface of the semiconductor is oxidized with oxygen atom radicals generated by irradiating an atmosphere containing an oxygen gas with light having a wavelength of 172 nm, the light emitted from a xenon excimer lamp.
Thus, the invention described herein makes possible the advantages of providing: (1) a method for forming an insulator film by high-rate oxidation without plasma damage, whereby a satisfactory interface is provided between a semiconductor and the gate insulator film, and a thick insulator film can be quickly and practically obtained; (2) a semiconductor device produced by the method; and (3) a production apparatus.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.